Monoenergetic gamma rays from nonminimal Kaluza-Klein dark matter annihilations

We investigate monoenergetic gamma-ray signatures from annihilations of dark matter comprised of $Z^1$, the first Kaluza-Klein (KK) excitation of the $Z$ boson in a nonminimal universal extra dimensions (UED) model. The self interactions of the non-Abelian $Z^1$ gauge boson give rise to a large number of contributing Feynman diagrams that do not exist for annihilations of the Abelian gauge boson $B^1$, which is the standard Kaluza-Klein dark matter (KKDM) candidate. We find that the annihilation rate is indeed considerably larger for the $Z^1$ than for the $B^1$. Even though relic density calculations indicate that the mass of the $Z^1$ should be larger than the mass of the $B^1$, the predicted monoenergetic gamma fluxes are of the same order of magnitude. We compare our results to existing experimental limits, as well as to future sensitivities, for image air Cherenkov telescopes, and we find that the limits are reached already with a moderately large boost factor. The realistic prospects for detection depend on the experimental energy resolution.

Figure 1

The annihilation rate for the process $Z^1{Z}^1\to \gamma \gamma$ as a function of the splitting between the LKP mass and the rest of the first-level excited KK spectrum, $\sqrt{\eta }=M_1/m_{\mathrm{LKP}}$.

Figure 2

The annihilation cross section for KKDM as a function of the mass, for the mass splitting $M_1/m_{\mathrm{LKP}}=1.1$. The solid parts of the curves indicate the mass ranges that give rise to the correct relic abundance for the respective DM candidates.

Figure 3

The predicted integrated flux above the threshold energy $E_{\gamma }$, with an experimental energy resolution given by $\sigma$, as a function of $E_{\gamma }$ for the $Z^1$ and the $B^1$, as well as the upper limits given by a number of experiments. The experimental data is taken from Refs. 44, 45.

Figure 4

The one-loop level Feynman diagrams involving internal fermion fields for the process $Z^1{Z}^1\to \gamma \gamma$. For each diagram, there is also a corresponding one with crossed final states, making the total number equal to six. The notation $A^{(0,1)}$ simply indicates that there are two different choices for the flow of KK number in the diagram, and the first or second choice should be used throughout the diagram.

Figure 5

The one-loop level Feynman diagrams involving internal gauge fields for the process $Z^1{Z}^1\to \gamma \gamma$. For each diagram in the first four rows, there is also a corresponding one with crossed final states, making the total number equal to 60. The notation $A^{(0,1)}$ simply indicates that there are two different choices for the flow of KK number in the diagram, and the first or second choice should be used throughout the diagram.

Figure 6

The one-loop level Feynman diagrams involving internal Higgs scalars for the process $Z^1{Z}^1\to \gamma \gamma$. Here, $\chi$ denotes the charged component of the Higgs doublet. For each diagram in the first row, there is also a corresponding one with crossed final states, making the total number equal to 22. The notation $A^{(0,1)}$ simply indicates that there are two different choices for the flow of KK number in the diagram, and the first or second choice should be used throughout the diagram.